ACCORDING to the Dictionary of Modern and Contemporary Art authored by Chilvers and Glades-Smith, to appropriate in art is to properly adopt, borrow, recycle or sample aspects (or the entire form) of pre-existing human-made objects of visual culture.
If we are to agree that this is an almost-exact definition of appropriation in art, then the word "properly" in that sentence is key. I think it's almost within the purview of common sense, anyway---as there is obviously an ocean of difference between, say, making an original joke, making a variation on a common or popular joke earlier made by the same or a different comedian, and making an appropriation (or grabbing) of a common or popular joke in order to twist its earlier context.
And what would a proper appropriation constitute?
First, we used the word "common" or "popular", for it is this element of commonness (or popularity) of a sonic or visual object that would place it within a society's aural or visual culture, wouldn't it, and that would allow for that object's appropriation by other artists to be possible. That is to say, one can appropriate a Mickey Mouse image without apologizing for the appropriation, since everybody knows where that image comes from and who it belongs to anyway. Appropriating an object not commonly known (not popular), in contrast, could be said to have committed thievery. Again, it is the commonness or popularity of the product, image or construct that allows an appropriation of it the regard of having operated within the bounds of propriety. Put another way, with this element of commonness or popularity, one needs not refer anymore to the original source in order to express one's appropriation or variation-making. One needs not state the obvious, as it were.
In contrast, regarding the "appropriation" of unknown products or objects, it is almost impossible to apply the propriety required above, the necessity of commonness or popularity being absent. However, while the above requires solely the commonness or popularity of the appropriated object-image in order for the propriety of its appropriation to be said to have transpired, the attachment of an acknowledgment could enhance the propriety of the implementation of the appropriation act. In such a case, instead of merely doing an appropriation of the Mickey Mouse image, one may have included Mickey Mouse's name in the title or subtitle of the work, or somewhere else. And this is where the appropriation of unknown products or object-images can actually find a resort. If proper appropriation, as in the above paragraph, could be enhanced by an acknowledgement (via a mention of the appropriated object in one's title or in an interview, is it possible then for the appropriation of an unknown object by an unknown creator to ensue simply through the insertion of an acknowledgement, regardless of the fact that the object of the appropriation is unknown to the world? I would say yes, it is possible, as an acknowledgment, any acknowledgment, need not be a mention of a common or popular object or author. by applying an acknowledgement of that unknown work by an unknown author as being the source of one's appropriation, one has already popularized the source to one's audience, thus rendering the requisite stated in the preceding paragraph already present. So, can one do such an acknowledging simply by titling their appropriation-ridden creation with something like "A Variation on an Apeldoorn Park Nude Sculpture (a Photo of Which Appears Below My Sculpture) Created By An Artist Not Yet Known to Many and Whose Name Escapes Me Even At the Moment"? Why not? [P-CA]